This invention relates to an electric melting furnace for glass-solidification of highly radioactive waste generated at a nuclear fuel reprocessing plant.
The waste generated at a nuclear fuel reprocessing plant ordinarily is a liquid and is highly radioactive. In order to safely isolate this waste from the human environment for an extended period of time, solidification technology has been developed for forming the waste and a glass material into molten glass by heating at high temperature, and sealing the glass in canisters in solidified form.
More specifically, the highly radioactive waste is subjected to suitable pretreatment, after which the waste, usually in the form of liquid, is fed into an electric melting furnace (hereinafter referred to as a "melter") together with the glass raw material. The highly radioactive waste and the glass material are formed into high-temperature molten glass within the melter. Metal vessels, namely the canisters referred to above, are filled with the molten glass continuously or intermittently. The canisters so charged with the glass are sealed and kept temporarily at a storage facility before being buried deep within the earth for permanent disposal.
The highly radioactive waste and the glass raw material (the combination of which shall hereinafter be referred to simply as the "raw material" where appropriate) continuously fed into the melter attain a state covering the molten glass surface of a melting cell made of brick. Owing to heat flow from the molten glass, evaporation of the water content in the waste, provisional combustion and a glass-forming reaction occur continuously, so that the raw material mixes with the already existing molten glass to form homogeneous glass.
The energy needed to maintain the molten glass at a high temperature is supplied by passing a current across at least a pair of opposing electrodes arranged in the molten glass to subject the molten glass between these electrode to Joule's heating.
In order to prevent the operator from being exposed to radiation, the melter is placed in a space referred to as a cell and operation, maintenance and exchange are performed by remote handling. Accordingly, the melter is designed so as to be as small in size and light in weight as possible. The conventional melter for technical development used in solidifying highly radioactive waste in glass also is designed so as to make the volume of the melting cavity as small as possible. That is, the depth of the melting cavity is made as small as the emplacement of the aforementioned electrodes will allow, and the bottom of the melting cavity is designed to be substantially horizontal to reduce the volume of the cavity.
Highly radioactive waste contains such elements of the platinum group as Ru, Pb and Rh. These elements do not readily dissolve in glass and have a high specific gravity. As a result, they deposit and built up on the bottom of the melting cavity. Among these elements of the platinum group, Pd and Rh are reduced in glass and are present as metals. Ru is present as a metal or as RuO.sub.2 crystal.
Though RuO.sub.2 is an oxide, it is known to be an excellent conductor and is a substance which finds use in electrically conductive pastes for electronic components. The metals of reduced Pd and Rh naturally are good electrical conductors. When these substances accumulate at the bottom of the furnace in high concentration, the high-temperature resistivity value of the glass near the bottom of the furnace is small in comparison with that of the glass at the upper part of the furnace. (The glass in the vicinity of the furnace bottom that contains the platinum-group elements in high concentration shall be referred to as "sludge" hereinafter.)
When the elements of the platinum group deposit on the furnace bottom and form an excellent electrically conductive layer in the melter of the conventional design having the shallow melting cavity, the electric current that flows between the electrodes concentrates at the bottom of the furnace, thereby causing an abnormal rise in the temperature at the furnace bottom and, conversely, a drop in the glass temperature at the surface of the melting cavity. The result is a decline in the efficiency at which the raw material is melted. Since the bottom surface of the melting cavity is substantially horizontal, moreover, the platinum-group elements that have deposited on the furnace bottom do not flow into the canister along with the glass and these continue to accumulate on the bottom until operation of the melter can no longer be maintained.
In order for highly radioactive waste containing platinum-group elements to be melted with glass in a stable manner by means of the melter using the Joule's heating method, it is necessary that the melter possess the following two functions:
(1) Since elements of the platinum group are not readily soluble in glass and exhibit a specific gravity of 10 or higher as opposed to the specific gravity (usually 2.5) of a glass melt, these elements quickly settle within the glass melt and deposit on the furnace bottom. The glass containing these platinum-group elements in high concentration, namely the aforementioned sludge, has a high-temperature resistivity lower than that of the glass at the upper portion of the furnace. Therefore, when the melting of the highly radioactive waste with glass begins, a highly electrically conductive layer forms in a short period of time.
Accordingly, it is required that the melter for solidifying the highly radioactive waste in glass be designed to have a structure in which operation is capable of continuing without impediment even if a highly conductive layer is present do some degree on the furnace bottom. In other words, the structure should be such that the current flowing between the electrodes does nor concentrate selectively at the bottom of the furnace.
(2) It is stated above in (1) that the melter design should be such that the electrode arrangement enables operation to continue even if a highly electrically conductive layer is present to some degree on the furnace bottom. However, if sludge remains inside the melting cavity when the glass is charged into the canister, and if the sludge keeps on accumulating, then this will naturally impede the heating caused by enerigization of the electrodes.
Accordingly, it is required to adopt a melter design in which the furnace bottom is inclined so that sludge will flow down to an outflow port in order that the deposit may be removed on a scheduled or non-scheduled basis.
Two patent applications relating to the furnace bottom configuration of a melting cavity are disclosed in Japanese Patent Application Laid-Open (KOKAI) Nos. 57-196726 and 57-19727. These two applications deal with the furnace bottom shape of an ordinary commercial glass melting furnace. They facilitate the changing of glass texture and provide a funnel-shaped bottom having an inclination of 3.degree.-45.degree. to prevent the furnace bottom refractory from being attacked by lead produced when lead glass is melted. Thus, both of these patent applications deal with the productivity and service life of a furnace but are silent with regard to electrode arrangement. Japanese Patent Application No. 60-275595 has as its object to prevent the electrical ill effects of the platinum-group elements. In order to prevent the concentration of electric current in the furnace bottom deposit, this application proposes making the distance between the glass outflow port opening in the furnace and the lower ends of at least a pair of electrodes, which are for supplying the major portion of the electric power necessary to melt the glass, no less than one-half the distance between the electrodes. If the glass melting capability of the melter (namely the amount of waste treated or the amount of glass manufactured per unit time) is to be increased, it is generally required that the area of the molten glass exposed at the top (which area shall be referred to as the "melt surface area" hereinafter) be increased. This is accompanied by an increase in the distance between the electrodes. Therefore, in accordance with the prior-art method disclosed in this patent application, the distance between the lower ends of the electrodes and the outflow port opening in the furnace is increased in proportion to the inter-electrode distance, and the depth of the melting cavity is increased as well. In other words, owing to the increase in the melting capability of the melter, there is an increase in the external dimensions of the melter and in the overall weight. This makes it necessary to furnish more space for installation and to provide a crane having greater capability for handling the melter in a melter facility for highly radioactive waste solidification. Moreover, if the melting cavity is provided with greater depth, the amount of glass to be heated will increase and it will be required to augment the heating apparatus.
The present invention seeks to solve the aforementioned problems and, to this end, partitions the portion of a melting cavity below the upper ends of the electrodes by means of an electrically non-conductive refractory (non-conductive partitioning refractory hereinafter), and makes the distance between the lower ends of the electrodes and an outflow port opening inside the furnace no less than one-half the distance between the partitioning refractory and the electrode closest thereto. This method of preventing the concentration of electric current in sludge differs from that of the abovementioned patent application. A glass outflow port is provided in each section of the melting cavity partitioned by the partitioning refractory, and a refractory having an incline is provided about the periphery of each outflow port. As a result, sludge flows out from the outflow port along the incline of the refractory, thereby preventing the deposits from building up. In accordance with the method of the invention, the melt surface area is enlarged to raise the treatment capability of the melter. Even if the inter-electrode distance is increased, melting cavity depth can be made one-half or more than one-half the electrode-partitioning refractory distance, which can be set at will. Thus, the depth of the melting cavity can be held below a fixed depth even if the treatment capability of the melter is raised. This makes it possible to reduce the external dimensions and total weight of the melter by an amount corresponding to the reduction in melting cavity depth. Furthermore, since the amount of retained glass to be heated can be reduced in comparison with the case when the method of the invention is not employed, the amount by which the heating apparatus must be augmented is kept small.
Further, in accordance with the invention, the glass melting furnace is provided with a plurality of glass outflow ports and freeze valves. By reducing the operating frequency per freeze valve, the load per freeze valve is decreased. This makes it possible to enhance the reliability and extend the service life of the overall glass melting furnace. Further, the partitioning refractory is equipped with a communication pipe. Thus, if a freeze valve should fail, the glass in the melting cavity can still be extracted by operation of the freeze valves that are not malfunctioning.